JP3561711B2 - Multilayer recording disk and recording / reproducing system using the same - Google Patents

Description

【００３６】
【数１４】

【００３７】
である。デフォーカス情報記録面の反射光成分がオンフォーカス情報記録面の反射光成分に対し十分抑圧されるためには、上述の如く少なくともＷ_２０ ^ｒｍｓ ＞０．２λが不可欠であるので、
【００３８】
【数１５】

【００３９】
【数１６】

【００４０】
【数１７】
∴ ｔ＞１．３９ｎλ／ＮＡ^２ …（１．１３）
となる。
ｎ＝１．５とすれば、
【００４１】
【数１８】
ｔ＞２λ／ＮＡ^２ …（１．１４）
である。
例えば、記録信号が７個以上０が連続しない０，１符号化による（１，７）変調符号で、最短反転間隔２Ｔ（Ｔはサンプル周期）が例えば規格化空間周波数で１．２、最大反転間隔８Ｔがその１／４の０．３であったとすると、図９より、デフォーカス収差量が０．６ｒｍｓλ以上あれば、１．２〜０．３の規格化空間周波数帯域において、オンフォーカスでの再生レベルに対しデフォーカスでの再生レベルが１／１０以下となることを充足する。Ｗ_２０ ^ｒｍｓ ＝０．６λとし、ｎ＝１．５，λ＝６８０ｎｍ，ＮＡ＝０．５５とすると、上式（１．１０）より、第１及び第２情報記録層間の厚みｔは、１４μｍ以上あれば良いことが分かる。
【００４２】
また、Ｗ_２０ ^ｒｍｓ ＝０．２λとし、同じくｎ＝１．５，λ＝６８０ｎｍ，ＮＡ＝０．５５とすると、上式（１．１０）より、第１及び第２情報記録層間の厚みｔには４．６７μｍの下限値が導かれるが、フォーカスサーボにおけるインフォーカスのキャプチャレンジを考慮すると、厚みｔは１０μｍよりも大きくさせることが望ましい。
【００４３】
すなわち、デフォーカス情報記録面からの反射光により、フォーカスエラー信号に誤信号を有することとなるが、これを十分小さくするには、図４に示される如きキャプチャーレンジＣＲの３倍程度必要である。通常キャプチャーレンジは、少なくとも３〜４μｍ以上は必要であるから、
【００４４】
【数１９】
∴ ｔ≧１０μｍ …（１．１５）
となる。
この式（１．１５）によって定められる下限値及び先の式（１．１０）に基づいて定められる下限値のうち値の大きい方を採用することが好ましい。
【００４５】
・上限値
情報記録面間の距離ｔが大きくなりすぎると、球面収差が増えて記録信号を読み取れなくなるため、厚さｔの上限値が導かれる。
すなわち、図１０の如きｉ（ｉは２以上の整数）個の情報記録面を有するディスクにおいて、第１情報記録面Ｓ_１ を読む場合と第ｉ情報記録面Ｓ_ｉ を読む場合では、光に対する厚みが（ｉ−１）ｔだけ異なる（ｔは記録面間距離、各記録面間は等距離）。このとき、球面収差が発生してしまう。
【００４６】
いま、第１情報記録面Ｓ_１ と第ｉ情報記録面Ｓ_ｉ との間の中間点に対して球面収差の補正されたレンズを用いて、第１情報記録面Ｓ_１ または第ｉ情報記録面Ｓ_ｉ を読む時ｉ個の各層の記録層内で発生する球面収差は、
【００４７】
【数２０】

【００４８】
となる。通常光ディスクの再生において、波面収差値はマレシャルクライラリオン（０．０７ｒｍｓλ）より小さくする必要があることが経験上知られている。波面収差には対物レンズの収差やディスク傾きによる収差も含めて考える必要があるので、Ｗ_４０ ^ｒｍｓの許容量は約０．０７ｒｍｓλの１／２以下にする必要があるので、
【００４９】
【数２１】

【００５０】
【数２２】
∴ ｔ ≦ ７．５ｎ^３λ／｛（ｎ^２−１）ＮＡ^４（ｉ−１）｝ …（１．１８）
である。
【００５１】
いま、ｎ≒１．５とすると、
【００５２】
【数２３】
ｔ ≦（２０／ＮＡ^４）｛λ／（ｉ−１）｝ …（１．１９）
となり、さらにｉ＝２とすれば、
【００５３】
【数２４】
ｔ≦２０λ／ＮＡ^４ …（１．２０）
となる。
故に（１．１４）式及び（１．２０）式によってｎ＝１．５，ｉ＝２の場合の情報記録面間の距離ｔは、
【００５４】
【数２５】
２λ／ＮＡ^２ ≦ ｔ ≦ ２０λ／ＮＡ^４ …（１．２１）
と定められる。この式に基づき、比較的採用しやすい実際上の値にてＮＡ及びλを変えてｔの下限及び上限値を求めたものが図１１の表に示される。さらにこの条件に（１．１５）式を加え、
【００５５】
【数２６】
１０μｍ ≦ ｔ ≦ ２０λ／ＮＡ^４ …（１．２２）
を充足することが好ましい。
かくして事項１をまとめると、既述の式（１．１３），（１．１８）の如く、
【００５６】
【数２７】
１．３９ｎλ／（ＮＡ）^２≦ｔ≦７．５ｎ^３λ／｛（ｎ^２−１）（ＮＡ）^４（ｉ−１）｝ …（Ｉ）
を満たす多層構造ディスクを作製することにより、デフォーカス情報記録面による影響を受けることなくフォーカス情報記録面の記録情報を読み取ることができる。そしてさらに好ましくは、上式に加えて、
【００５７】
【数２８】
１０μｍ≦ｔ≦７．５ｎ^３λ／｛（ｎ^２−１）（ＮＡ）^４（ｉ−１）｝ …（ＩＩ）
を満たすことにより、フォーカスサーボにおける引き込み動作可能幅であるキャプチャレンジに基づくフォーカスエラー信号の保全をなすことができるのである。
【００５８】
なお、事項１及び２では、情報記録面の各々が異なる反射光帯域を有することが要求されない。
次に、上記事項３について説明する。
これは、入射する読取光の波長により反射率や透過率が変化しない膜を各情報記録面の反射膜として用いる場合に適用される。
【００５９】
例えば、図１において、第２情報記録面（反射層１５）の読み取りにおいては、第１情報記録面（反射層１２）を透過してきた光が第２情報記録面で反射し、この反射した光がさらに第１情報記録面を透過して、ディスクの反射光として光源側に戻ってきた光が受光され、その受光量に応じた読取信号が得られる。各反射層は、光を吸収しないものとして、第１情報記録面の反射率をα，透過率を（１−α）、第２情報記録面の反射率をβ，透過率を（１−β）とし、読取光量をＰとし、第２情報記録面を透過した光はほぼディスクの反射光として戻ってこないと仮定し、各情報記録面のピットによる変調度をｋとすると、第１情報記録面を読む際に得られる読取信号の変調度Ｍ１は、
【００６０】
【数２９】
Ｍ１＝Ｐαｋ／｛α＋（１−α）^２β｝
＝Ｐαｋ／（α＋β−２αβ＋α^２β）
≒Ｐαｋ／（α＋β） …（３．１）
第２情報記録面を読む際に得られる読取信号の変調度Ｍ２は、
【００６１】
【数３０】
Ｍ２＝Ｐ（１−α）^２βｋ／｛α＋（１−α）^２β｝
≒Ｐ（１−α）^２βｋ／（α＋β） …（３．２）α，βともに１より小さい数であり、従って（１−α）^２は、１より小さい数の２乗値である。従って（３．１）式と（３．２）式が同じような値になるためには、βはαより十分大きな必要がある。すなわち、読取光が入射する側から見て、遠い情報記録面の反射率は、近い情報記録面の反射率より大きく設定する必要がある。
【００６２】
例えば、α＝０．３６，β＝０．９とすると、
【００６３】
【数３１】
Ｍ１≒０．３６Ｐｋ／１．２６ …（３．３）
【００６４】
【数３２】
Ｍ２≒０．３７Ｐｋ／１．２６ …（３．４）
というように、Ｍ１とＭ２はほぼ等しくなる。この場合の各情報記録面における反射率特性を示したものが図１２であり、横軸λが読取光の波長を表しており、縦軸Ｒはその読取光による反射率を表している。
【００６５】
なお、事項３でも、情報記録面の各々が異なる反射光帯域を有することが要求されない。
次に、上記事項４について説明する。ここでは、情報記録面の各々が異なる反射光帯域を有することが要求される。
上記図１２とともに説明した実施例では、単一波長の読み取り光源を用いて複数の情報記録面を読み取るときの反射膜の条件を示したが、理想的には、１つの情報記録面を読み取るときには、他の情報記録面は無色透明であることが望ましい。読み取り光源の波長を複数にし、各反射膜にこれら波長にそれぞれ対応して反射するようなものを採用すると、これに近い状態とすることができる。しかし、完全に無色透明というわけにはいかないので、この場合でも、上記式（Ｉ）を、好ましくは（ＩＩ）を満たすことが望ましい。
【００６６】
従って、当該式（Ｉ）または（ＩＩ）を満たした上で、図１３に示されるように、第１情報記録面Ｓ_１ は、波長λ_１ の読取光で、第２情報記録面Ｓ_２ は波長λ_２ の読取光で、第３情報面Ｓ_３ は、波長λ_３ の読取光で読み取るものとする。すなわち、図１４に示されるように、第１情報記録面の反射膜は、波長λ_１ で大きな反射率を示し、λ_２，λ_３では、反射率は落ちて、透過率が上がり、第２情報記録面の反射膜は、波長λ_２ で大きな反射率を示し、λ_１，λ_３では、反射率は落ちて、透過率が上がり、第３情報記録面の反射膜は、波長λ_３ で大きな反射率を示し、λ_１，λ_２では、反射率は落ちて、透過率が上がるような特性に設定するのである。これにより、Ｓ／Ｎの良好な読取信号が得られる。なお、このような反射膜には、例えば、色素材料等を用いて、波長選択性の有る光反射材が採用される。
【００６７】
次に、上記事項５について説明する。ここでは、複数の情報記録面のうち読取光の入射側に近いほうの情報記録面が所定の反射率特性を有することが要求される。
上記図１３及び図１４とともに説明した実施例では、読み取り光が、複数の波長を有する場合を示したが、読取光の波長を１種類とし、当該読取光以外の光源を用いて情報記録面の反射膜の反射率特性を変えつつ記録情報を読み取るようにしても良い。
【００６８】
例えば、フォトクロミック材料の中には、図１５のように、波長λ_２ の単独の入射光では、反射率に変化がないが、波長λ_２ の入射光に加えて波長λ_１ の光を照射すると、波長λ_２ 及びこの前後の波長において反射率が上がるものがある。そこで、図１６に示すように、透明基板１１に近い方の情報記録面の反射膜１２Ｂを、フォトクロミック材料とする。例えば、読み取り光源を赤色レーザダイオードとし、付加光として緑色の光源を照射した時には、情報記録面であるフォトクロミック層が反応して、赤色での反射率が上がるようなディスクを作製する。
【００６９】
かかるディスクの記録情報再生システムとしては、図１７のように、赤色のレーザービーム読取光をディスクに照射して情報検出をなすピックアップ３１は１個だけ設けられ、このピックアップとは別に緑色光を発射するランプ３２が設けられる。ランプ３２は、第１情報記録面（１２Ｂ）を読むときは、上述の如く作製されたディスク３３（少なくともピックアップ３１の情報検出点及びその近傍）を照射すべく点灯され、第２情報記録面（１５）を読むときは緑色ランプ３２を消灯する。
【００７０】
また、図１８のように、ピックアップ３１Ａ内部に緑色の発光ダイオード３１０を有し、このダイオード出力光と情報検出出力を得るための赤色のレーザーダイオード３１１により発射された赤色の読取光とを合成してディスク３３に照射するようにしても良い。
図１８において、情報検出用赤色レーザ３１１の発射光は、緑色反射板３１２を透過してハーフミラー３１３及び対物レンズ３１４を介してディスク３３に入射する。ディスク３３から反射した赤色光は、ハーフミラー３１３を反射して赤色フィルタ３１５及び光学系３１６を介しフォトディテクタ３１７に導かれる。かかる光学系３１６は、例えば平行平板及びレンズからなり、フィルタ３１５を透過した光をフォトディテクタ３１７へ導き集光する。一方、付加光源たる緑色発光ダイオード３１０の発射光は、レンズ３１８を介して緑色反射板３１２に入射しかつここで反射され、反射板３１２を透過したレーザ３１１からの赤色光とともにディスク３３へ入射する。
【００７１】
第１情報記録面を読み取るときは、駆動回路により緑色ダイオード３１０を発光させ、ディスク３３に赤色及び緑色の両波長光を照射して図１５の点線の特性の状態で読取動作をなす。第２情報記録面を読み取るときには、緑色ダイオード３１０を消灯させ、ディスク３３に赤色の波長光のみを照射して図１５の実線の特性の状態で読取動作をなす。従って、緑色ダイオード３１０は、読み取るべき情報記録面に対応してオンオフ駆動される。
【００７２】
さらに、かかるピックアップ３１Ａの如き構成のピックアップを複数用いて高読取転送レートの再生システムとしても良い。例えば当該ピックアップを２つ用意し、一方のピックアップを第１情報記録面の読み取り用に、他方のピックアップを第２情報記録面の読み取り用に、それぞれの赤色レーザ及び緑色ダイオードを動作させる。これにより、両情報記録面を同時に読み取ることができ、単位時間当たり沢山の情報を読み出すことができるので、大量の記録情報を扱うシステムに好適である。かかる構成では、必要に応じて一方と他方のピックアップの読み取る情報記録面を反対にすることも可能である。
【００７３】
なお、読取光及び付加光は赤色光及び緑色光であるとして説明したが、可視光でなくても、赤外や紫外光であっても良いし、場合によっては、電磁波やＸ線等で特性の変わるものであっても良い。また、付加光源としてはフォトダイオードやランプに限られることなく、レーザダイオードであっても良い。なお、図１８におけるダイオード３１０の発射光は、レーザほど絞ってディスク３３の情報記録面に照射する必要はないので、レンズ等の光学部品には負荷とならない。また、フォトディテクタ３１７には、記録情報の読み取り光成分だけを入射した方が望ましいので、赤色光のみを透過するフィルタ３１５がその前段に介挿され、読取効率の向上が図られている。
【００７４】
次に、上記事項６について説明する。ここでは、情報記録面の各々が異なる反射光帯域を有することを要求しない。
上記図１５ないし図１８とともに説明した実施例では、読取光に付加光を照射することにより反射率特性の変わる反射層を用いたが、図１９のように、透面電極に挟まれた液晶層により各情報記録層における反射率を変えるようにしても良い。
【００７５】
図１９において、透明基板１１には第１の情報記録層として透明かつ導電性の反射層５１が積層されるとともに、この反射層５１は、ディスク５０の外表面に例えば最外周近傍に配された導体リング５１ｒと電気的に導通するよう接続される。保護層１６に積層された第２の情報記録層としての反射層１５及び透明樹脂層１４上には透明電極層５２が形成される。かかる一対の透明電極層５１，５２の間にはスペース部５３に囲繞された液晶層５４が形成される。導体リング５２ｒは、導体リング５１ｒよりも内周側に離間して配され、各リングは、露出している。
【００７６】
このディスク５０を読取再生するときには、各リングの露出面にブラシを接触させつつ当該ディスクを回転させる。そうして、第１情報記録面を読み取るときには、駆動回路５７によって、一方の導体リング５１ｒに接触するブラシ５５と、他方のリング５２ｒに接触するブラシ５６との間に液晶層５４を暗とすべき駆動電圧を印加しながら読取光を入射する。これにより、暗となった液晶層５４によって第２情報記録層１５への読取光の入射が遮断されるので、第１情報記録面のみが読み取られることとなる。第２情報記録面を読み取るときには、駆動回路５７がオフとされ液晶層５４が明となるので、読取光が第２情報記録層に到達し、第２情報記録面が読み取れる。
【００７７】
かかる構成によって、第１情報記録面の読み取り時に、第２情報記録面へ入射する読み取り光がなくなるので、第２情報記録面の記録信号による不要な反射光成分を生じることなく第１情報記録面を読み取ることができ、さらなるＳ／Ｎの向上が達成される。
次に、事項７について説明する。
【００７８】
単一のピックアップで複数の情報記録面の読み取りをする場合は特に、次の点に鑑みるべきである。
ディスクの１の情報記録面において、例えばスパイラルトラック状に内周から外周へと信号が記録され、当該情報記録面に記録された最後の信号が別の記録情報記録面に続くものとすると、各情報記録面におけるスパイラルの向きが同じであり、ディスクの回転方向を変えないならば、図２０に示されるように、当該最後の信号に続くべく別の情報記録面の読取動作に移行するときに、内周に戻って、フォーカスを取り直す必要がある。また、ＣＬＶであるならば、ディスクの回転数を極端に変えなければならない。これに対し当該最後の信号の読取後、そのまま外周でフォーカスを取り直して、別の情報記録面の読取動作を外周から行うためには、ディスクを逆回転にしなければならない。
【００７９】
かかる不具合に鑑みれば、図２１に示されるように、例えば、第１情報記録面と第２情報記録面とで記録トラックのスパイラル方向を逆とし、第１情報記録面で内周から外周へと信号を読み取り、最後の信号を読み取った後は、ディスクの回転方向も回転数も変えずにフォーカスを取り直し、そのまま第２情報記録面の読取動作に移行して外周から内周へと読み取ることが好ましい。こうすることにより、多層構造ディスクの読取動作の円滑な連続性を保つことができる。情報記録層が３層あるディスクであれば、少なくとも１層が逆スパイラルになっていれば、かかる連続性が保てることになる。
【００８０】
なお、情報記録層が２つある場合、フォーカスの取りやすさを考慮すれば、第１情報記録面における読み取りは、外周スタートで、内周に進み、当該情報記録面の最後の信号を読み取った後は、フォーカスを取り直して、第２情報記録面の読み取りを内周から外周に進ませるのが良い。また、スパイラルトラックに限らず同心円トラックにも本事項は適用可能である。
【００８１】
次に、上記事項８について説明する。ここでは、再生系のランダムアクセスの容易性を考慮している。
【００８２】
複数の情報記録面のうちの１つをイニシャル面とし、このイニシャル面の所定の記録領域にディスクの全記録情報についてのアロケーション情報（通常のＣＤで使用されているようなＴＯＣ情報に等価な情報，以下、ＴＯＣ情報と呼ぶ）を記録する。これを示したのが図２２であり、例えば第１情報記録層をイニシャル面に定め、その記録領域の先頭部にＴＯＣ情報を記録する。かかるＴＯＣエリアには、第１から第ｍまでの当該ディスクの全ての情報記録層についてのＴＯＣ情報が個別に記録される。
【００８３】
このように記録されたＴＯＣ情報を読取動作の立ち上げの際に予め読み取っておくことにより、後に指示されるランダムアクセスに機敏に応答することができる。
他の方法としては、図２３に示されるように、情報記録層の各々にＴＯＣエリアを設け、このエリアに第１から第ｍまでの当該ディスクの全ての情報記録層についてのＴＯＣ情報を記録するようにしても良い。これによれば、イニシャル面を特に定めなくとも、どの情報記録面からでもディスク全体についてのＴＯＣ情報を読み取ることができる。
【００８４】
事項９の説明の前に、先の図１０を参照し、情報記録面Ｓ_ｉ を読み取るべく基板１１及び情報記録面Ｓ_１ 〜Ｓ_ｉ−１ を読取光が透過し情報記録面Ｓ_ｉ に焦点制御された場合（Ａ）を考える。この場合、情報記録面Ｓ_ｉ で反射した戻り光は、再び情報記録面Ｓ_１ 〜Ｓ_ｉ−１ を透過し基板１１を経て図示せぬ受光系に導かれる。ここで、最下層の情報記録面Ｓ_ｉ の反射率が１００％、基板１１と情報記録面Ｓ_１ との間の透過率及び各情報記録面間の透過率をそれぞれαとすると、基板１１から出る戻り光の光量は、α^２ｉで表される。例えば当該透過率が９０％で情報記録面の積層数が１０である場合はα＝０．９、ｉ＝１０であるので、ディスクへ入射する読取光量１００％に対し僅か１２％の戻り光量しか得られない。この試算においては情報記録面Ｓ_ｉ の反射率を１００％にしたが、情報記録面Ｓ_ｉ が他の情報記録面と同様の光反射及び透過特性を有する場合、基板１１から出る戻り光の光量は、α^ｉ−１（α−１）α^ｉ−１で表される。この場合にα＝０．９、ｉ＝１０とすると、ディスクへ入射する読取光量１００％に対しさらに少ない１．５％の戻り光量しか得られないことになる。このように、記録面の多層構造においては、最下層の情報記録面まで読取光を通過させるために中間の各情報記録層の透過率を高くする必要がある一方、情報記録層の透過率を高くすると読み取るべき情報記録面において読取光を十分に反射できず十分な光量の戻り光が得られないという矛盾がある。
【００８５】
そこで読み取るべき情報記録面すなわち読取光の焦点制御がなされている情報記録面についてのみ十分な戻り光量を得る方法を模索した結果、読取光のエネルギー密度が情報記録面でそれぞれ異なり、しかも読取光の焦点制御がなされている情報記録面では読取光のエネルギー密度が最大であることに着目した。例えば、情報記録面間距離ｔ＝２００μｍ，対物レンズの開口数ＮＡ＝０．５，情報記録面間の屈折率ｎ＝１．５とすると、情報記録面Ｓ_３ に読取光の焦点が合わせられている場合（Ｂ）に、情報記録面Ｓ_２ での読取光スポットの直径は、近似計算で２００÷１．５×０．５×２＝１３２［μｍ］となる。合焦点での読取光スポットは約１μｍであるので、情報記録面Ｓ_２ における読取光のエネルギー密度は情報記録面Ｓ_３ の１／１００以下となり、結局、隣合う情報記録面において読取光のエネルギー密度に相当な差があることが分かる。
【００８６】
本事項においては、かかる情報記録面間における読取光のエネルギー密度の相違を利用し、情報記録面Ｓ_１ 〜Ｓ_ｉ を形成する反射層に、光エネルギー密度に応じた反射率を示す材料を採用する。つまり、読取光が合焦し光エネルギー密度が最大となった場合に反射率が著しく上昇しこれ以外の場合では比較的高い透過率を持つような性質の光反射材にて情報記録膜を形成するのである。このような情報記録膜としては、フォトクロミック膜、サーモクロミック膜、もしくは相変化膜等が挙げられる。かかる情報記録膜にて各情報記録面が形成された多層記録ディスクに対しては、単一の読取光で所望の情報記録面の読み取りを行うことができる。読取光の合焦位置にある情報記録面では、上記の性質により入射した読取光を高い反射率で反射させる一方、読取光の合焦位置にない情報記録面では入射した読取光を高い透過率で透過させるので、ディスクの戻り光には合焦位置にある情報記録面により変調された成分が多分に含まれ、合焦位置にない情報記録面により変調された成分は極めて少ない。従ってこのような戻り光から読み取るべき情報記録面の読取信号を良好に得ることができるのである。
【００８７】
また、本事項においては、かかる情報記録面間における読取光のエネルギー密度の相違を利用し、情報記録面Ｓ_１ 〜Ｓ_ｉ を形成する反射層に、光エネルギー密度に応じた反射帯域を示す材料を採用する。つまり、読取光が合焦し光エネルギー密度が最大となった場合に読取光とは異なる波長成分を反射しこれ以外の場合では読取光と同等の波長成分を反射するような波長変換をなす性質の光反射材にて情報記録膜を形成するのである。このような情報記録膜としては、フォトルミネッセンス膜、蛍光色素膜、光半導体膜等が挙げられる。かかる情報記録膜にて各情報記録面が形成された多層記録ディスクに対しても、単一の読取光で所望の情報記録面の読み取りを行うことができる。読取光の合焦位置にある情報記録面では、上記の性質により入射した読取光とは異なる第２波長の光を反射させる一方、読取光の合焦位置にない情報記録面では入射した読取光と同じ第１波長の光を反射させるので、ディスクの戻り光には合焦位置にある情報記録面により変調された第２波長光と、合焦位置にない情報記録面により変調された第１波長光とが含まれる。従ってこのような戻り光から第２波長光を抽出しこれを読み取るべき情報記録面の読取信号として得ることができるのである。
【００８８】
第２波長光を抽出する手段を含む再生システムの一例は、図２４に示されており、図１８と同等の部分には同一の符号が付されている。
図２４において、上述の如き情報記録面Ｓ_１ ，Ｓ_２ ，Ｓ_３ を形成する反射層に光エネルギー密度に応じた反射帯域を示す材料が採用された光ディスク３３´には、光源３１１´からの波長λ１ のレーザビームが読取光としてハーフミラー３１３及び対物レンズ３１４を介して照射される。この光ディスク３３´への読取光が真ん中の情報記録面Ｓ_２ に合焦するようフォーカス制御されると、情報記録面Ｓ_２ における光エネルギー密度が増大し、情報記録面Ｓ_２ に入射した波長λ１ の読取光の一部成分はここで波長λ_２ の反射光に変換される。同時にこの波長λ１ の読取光の他の成分は情報記録面Ｓ_１ ，Ｓ_３ に達するが、ここでは光エネルギー密度が極めて低いので波長λ１ のまま読取光が反射する。かくしてディスク３３´からは波長λ１ の戻り光と波長λ_２ の戻り光が得られ、これらが対物レンズ３１４及びハーフミラー３１３を経て光学フィルタ３２０に導かれる。フィルタ３２０は、波長λ_２ の光成分のみを透過する抽出手段を担い、ハーフミラー３１３からの波長λ１ の戻り光を遮断し、波長λ_２ の戻り光のみをフォトディテクタ３１７へ供給する。従って、ディテクタ３１７は、読み取るべき情報記録面Ｓ_２ の記録信号によって変調された戻り光だけを受光することができる。ディテクタ３１７は、その受光量に応じた電気信号を読取信号として図示せぬ信号処理系に供給する。
【００８９】
なお図２４では波長の変化の様子を明瞭に表すために実際とは異なる形で模式的に光路が描かれている。また、対物レンズ３１４やハーフミラー３１３には、波長λ１ 及び波長λ_２ の双方について同等の光学特性を有するものを採用するのが好ましい。さらに、光エネルギー密度に応じた波長変換をなす情報記録層を有するディスクとしては、上述した如き反射型に限らず透過型のものにも適用することができる。
【００９０】
【発明の効果】
以上詳述したように、本発明の多層記録ディスクによれば、読取光が情報記録面のうちの１つにオンフォーカスしているときに得られる再生レベルに対し読取光が情報記録面のうちの他の情報記録面にデフォーカスしているときに得られる再生レベルが１／１０以下となる周波数帯域で情報記録面の各々に情報信号が記録されており、情報記録面各々の間の距離ｔ、当該読取光を情報記録面のいずれかに導く対物レンズの開口数ＮＡ、読取光の波長λ、情報記録面各々の間の等価屈折率ｎ、及び前記情報記録面の積層数ｉが、
【００９１】
【数３３】
1.4nλ／（NA）^２ ≦ ｔ ≦ 7.5n^３λ／｛（n^２-1）（NA）^４（i-1）｝
を充足し、情報記録面各々の内の少なくとも１の情報記録面には螺旋状に形成された記録トラック上においてディスク外周から内周に向けて情報記録が為されており、他の情報記録面には前記１の情報記録面に形成されている記録トラックとは異なる螺旋方向にて螺旋状に形成された記録トラック上においてディスク内周から外周に向けて情報記録が為されている。これにより、多層記録ディスクからの円滑な連続読取が可能になると共に所定値以上のデフォーカス収差及び所定値以下の球面収差が維持されるので、さらなる情報記録密度の向上を図り、しかも良好に信号を記録しかつその記録された信号を読み取り再生することのできる多層ディスク及びこれを用いた記録システム並びに再生システムを提供することができる。
【図面の簡単な説明】
【図１】本発明による実施例における、情報記録層として２つの光反射層を有するＲＯＭ型光ディスクの構造を示す断面図。
【図２】図１における光ディスクの作製方法を説明するための各工程におけるディスク断面図。
【図３】図１における光ディスクと等価な構造を有するＲＯＭ型光ディスクの構造を示す断面図。
【図４】図１及び図３の光ディスクに対するフォーカシング動作において、対物レンズとディスクとの間の距離の変位量に対するフォーカスエラーレベルを示すグラフ。
【図５】図１及び図３の光ディスクにおいて光ビームが第２情報記録面に合焦しているときの、第１情報記録面での光ビームの大きさを示す幾何学図。
【図６】図１及び図３の光ディスクにおいて光ビームが第１情報記録面に合焦しているときの、第２情報記録面での光ビームの大きさを示す幾何学図。
【図７】図１及び図３の光ディスクにおいて、透明基板と第１及び第２情報記録面間の層との屈折率が同一でない場合の、光ビームの第２情報記録面合焦時における第１情報記録面上光ビームの大きさを示す幾何学図。
【図８】図１及び図３の光ディスクにおいて、光ビームのデフォーカス及びオンフォーカス状態のＭＴＦ特性を示すグラフ。
【図９】デフォーカス時の再生レベルをオンフォーカス時の再生レベルの１／１０以下とした場合の、デフォーカス収差量に対する規格化空間周波数の最大値及び最小値を示すグラフ。
【図１０】本発明による光ディスクにおける情報記録面間の距離の上限値を導く説明及び読取光の各情報記録面における光エネルギー密度の相違の説明をするための、ｉ個の情報記録面を有するディスクの簡略的断面図。
【図１１】本発明による光ディスクにおける情報記録面間の距離の上限及び下限値の計算例を示す図表。
【図１２】本発明による光ディスクの、読取光の入射する側に近い一方の情報記録面、及び他方の情報記録面の反射率特性を示すグラフ。
【図１３】第１ないし第３情報記録面をそれぞれ異なる波長の読取光で読み取る様子を示すディスクの簡略的断面図。
【図１４】図１３における光ディスクの各情報記録面の反射率特性を示すグラフ。
【図１５】フォトクロミック材料の反射率特性を示すグラフ。
【図１６】本発明による実施例の多層記録ディスクを示す断面図。
【図１７】図１６におけるディスクの記録情報を再生する本発明一実施例の再生システムの構成を示すブロック図。
【図１８】図１６におけるディスクの記録情報を再生する本発明他の実施例の再生システムの構成を示すブロック図。
【図１９】本発明による実施例の、液晶層を有する多層記録ディスクの断面図及び外観図。
【図２０】多層記録ディスクにおける各情報記録面の記録トラックのスパイラル方向の不具合を説明するための模式図。
【図２１】本発明による多層記録ディスクにおける各情報記録面の記録トラックのスパイラル方向を説明するための模式図。
【図２２】本発明による多層記録ディスクにおけるＴＯＣ情報の記録形態の一例を示す模式図。
【図２３】本発明による多層記録ディスクにおけるＴＯＣ情報の記録形態の他の例を示す模式図。
【図２４】本発明による多層ディスクの記録信号を読み取る再生システムの構成を示すブロック図。
【主要部分の符号の説明】
１１，１１Ａ 透明基板
１２，１２Ａ 第１反射層
１３ 放射線透過性透明スタンパ
１４ 紫外線硬化型樹脂層
１５ 第２反射層
１６ 保護層
Ｓ_１ 〜Ｓ_ｉ 情報記録面
１２Ｂ フォトクロミック反射膜
３１，３１Ａ，３１Ａ´ ピックアップ
３１０ 緑色光ＬＥＤ
３１１ 赤色レーザ
３１２ 緑色反射板
３１３ ハーフミラー
３１４ 対物レンズ
３１５ 赤色フィルタ
３１６ 光学系
３１７ フォトディテクタ
３１８ レンズ
５０ 多層記録ディスク
５１，５２ 透明電極層
５１ｒ，５２ｒ 導体リング
５３ スペース部
５５，５６ ブラシ
５７ 駆動回路
３１１´ 読取光源
３２０ 光学フィルタ[0001]
[Industrial applications]
The present invention relates to a disk-shaped recording medium, and more particularly to a multilayer disk formed by laminating a plurality of information recording layers. In addition, the present invention relates to a system for recording a signal on each of the information recording layers, and a system for reproducing a signal recorded on each of the information recording layers.
[0002]
[Prior art]
Conventionally, increasing the density of information recorded on an optical disk has focused on increasing the recording density on the disk recording surface. Attempts have been made to reduce the track pitch or to increase the recording density in the linear velocity direction in recording and reading scanning in combination with shortening of the wavelength of a light source that emits a recording beam and signal processing of a reproducing system.
[0003]
However, even if the wavelength of the light source is shortened, it is limited to the ultraviolet region, and even if it is a pit, it can only be reduced to a size that can be transferred to a disk at the time of cutting. However, in the two-dimensional area of the disk, the limit will eventually come.
A multi-layer disc formed by laminating information recording layers in order to increase the density of recorded information in three dimensions, that is, in the thickness direction of the disc, is disclosed in Japanese Patent Application Laid-Open No. H2-223030 (Japanese Patent Application No. 1-20999). ) Etc.
[0004]
However, it has been found that it is insufficient to further improve the information recording density, record a signal well, and read and reproduce the recorded signal.
[0005]
[Problems to be solved by the invention]
The present invention has been made in view of the above points, and aims to further improve the information recording density, and furthermore, to record a signal well and to read and reproduce the recorded signal and a multilayer disc. It is an object of the present invention to provide a recording system and a reproducing system used.
[0006]
[Means for Solving the Problems]
A multilayer recording disk according to the present invention is a multilayer recording disk in which a plurality of information recording surfaces from which information is read only from one surface side of the disk surface are stacked, and a distance between each of the information recording surfaces. t, a numerical aperture NA of an objective lens for guiding read light for reading recorded information from the multilayer recording disk to any of the information recording surfaces, a wavelength λ of the read light, an equivalent refractive index n between each of the information recording surfaces, And the number i of stacked information recording surfaces is
[0007]
[Equation 3] 1.4nλ / (NA) ² ≦ t ≦ 7.5n ³ λ / {(n ² −1) (NA) ⁴ (i−1)}
Satisfies
The read light is defocused on another information recording surface of the information recording surface with respect to a reproduction level obtained when the read light is on-focused on one of the information recording surfaces. An information signal is recorded on each of the information recording surfaces in a frequency band in which the obtained reproduction level is 1/10 or less, and at least one of the information recording surfaces has a spiral shape. Information is recorded from the outer circumference of the disk to the inner circumference on the recording track formed in the shape of a circle, and the other information recording surface has a spiral direction different from that of the recording track formed on the one information recording surface. Is characterized in that information is recorded from the inner circumference to the outer circumference on the spirally formed recording track .
[0008]
[Action]
According to the multilayer recording disk of the present invention, the read light is applied to the other information recording surface of the information recording surface with respect to the reproduction level obtained when the read light is on-focused on one of the information recording surfaces. An information signal is recorded on each of the information recording surfaces in a frequency band in which a reproduction level obtained when defocusing is 1/10 or less. The numerical aperture NA of the objective lens to be guided to any of the recording surfaces, the wavelength λ of the reading light, the equivalent refractive index n between the information recording surfaces, and the number i of stacked information recording surfaces are:
[0009]
(Equation 4)
^{1.4nλ / (NA) 2 ≦ t} ≦ 7.5n 3 λ / {(n 2 -1) (NA) 4 (i-1)}
And at least one information recording surface of each of the information recording surfaces has information recorded on the spirally formed recording track from the outer periphery of the disk to the inner periphery thereof. The information is recorded from the inner circumference to the outer circumference of the disk on a spirally formed recording track in a different spiral direction from the recording track formed on the first information recording surface . This enables smooth continuous reading from the multilayer recording disk, and maintains a defocus aberration of a predetermined value or more and a spherical aberration of a predetermined value or less.
[0010]
【Example】
Hereinafter, the present invention will be described in detail with reference to the drawings.
A ROM optical disk having two light reflecting layers as information recording layers as shown in FIG. 1 is manufactured as follows.
First, as shown in FIG. 2A, a transparent substrate 11 carrying first recording pits P1 is prepared. The transparent substrate 11 is formed by using a nickel stamper (not shown) in which first recording pits P1 are spirally or concentrically arranged on the surface thereof as a molding die, and is made of a transparent resin such as PMMA (polymethacrylate) or PC (polycarbonate). Is a transparent substrate obtained by injection molding. By injection molding, the arrangement pits of the nickel stamper are transferred onto the substrate 11 as emboss pits, that is, first pits P1. As another method, the transparent substrate 11 may be manufactured by embossing pits on a glass substrate by etching.
[0011]
Next, as shown in FIG. 2B, silicon (Si), aluminum (Al), or various materials to be described later are deposited on the surface of the substrate 11 supporting the pits P1 using a vacuum deposition apparatus. Thus, the light reflection layer 12 is formed. Thus, the first reflection layer 12 is formed on the transparent substrate 11.
Next, a transparent stamper 13 is prepared and mounted on a transfer device. The transparent stamper 13 is made of radiation-transmissive glass or plastic that transmits light in a wavelength band necessary for curing a radiation-curable resin (ultraviolet-curable resin) 14 applied in the next step. Second recording pits are arranged spirally or concentrically on the surface of the transparent stamper 13, and as shown in FIG. 2 (c), the pit surface is mounted on the transfer device with the pit surface facing upward.
[0012]
Next, as shown in FIG. 2C, a liquid radiation-curable resin 14 is supplied onto the pit surface of the transparent stamper 13.
Next, the transparent substrate 11 is placed on the pit surface of the transparent stamper 13 via the liquid radiation curable resin 14 with the first reflection layer 12 facing downward. Thus, the radiation-curable resin 14 is held between the first reflective layer 12 and the transparent stamper 13 (FIG. 2D).
[0013]
Next, in the state shown in FIG. 2D, radiation is irradiated from the side of the transparent stamper 13, that is, from below the drawing, to cure the radiation-curable resin 14 on the first reflective layer 12. In this manner, the arrayed pits of the transparent stamper 13 are transferred onto the cured radiation-curable resin 14 carried as the second pits P2.
Next, as shown in FIG. 2E, after the radiation-curable resin is cured, the transparent stamper 13 is separated from the radiation-curable resin layer. Such a resin layer 14 can be formed with a thickness of several tens of μm. In addition, other materials may be used as long as they form the same layer, not necessarily the ultraviolet curable resin.
[0014]
Next, as shown in FIG. 2 (f), using a vacuum evaporation apparatus, silicon, aluminum, or various kinds of materials described later are formed on the surface of the radiation-curable resin 14 on which the embossed pits, that is, the pits P2, are carried. Is deposited to form the light reflection layer 15. Thus, the second reflection layer 15 is formed on the radiation-curable resin 14 by lamination.
[0015]
Next, as shown in FIG. 2G, a protective layer 16 made of a radiation curable resin for protecting the second reflective layer 15 is laminated, and the first and second reflective layers 12 and 15 having required characteristics are formed. Obtain a laminated optical disk.
Between the reflective layer 12 (or 15) and the transparent resin layer 14, or between the reflective layer 15 and the protective layer 16, there may be another protective film that transmits light. Further, as in the steps (f) to (c), (d) and (e), the transparent stamper, the radiation-curable resin and the light reflecting material are repeatedly used to form three or more reflecting layers ( Recording surface).
[0016]
In such a manufacturing method, the transparent substrate is provided with embossed pits for forming the first pits. However, as shown in FIG. 3, a flat glass substrate 11A is used, and a thin 2P layer of about 10 μm (ultraviolet rays) is used. The cured resin layer 14 may have embossed pits. In this case, the interface between the reflective layer 12A and the resin layer 14 becomes the first information recording surface.
[0017]
As for the disk as described above, the recording density of the disk increases as the number of laminated information recording layers increases. However, in order to simplify the description, an example of a ROM optical disk having two information recording surfaces will be described below. The required items of the optical disk of the present invention will be described.
As shown in FIG. 1, reading light for detecting pits as recording information is emitted from the outer surface side of the transparent substrate 11 toward the information recording surface, and the recording information on the reflection layer 15, that is, the second information recording surface is read. Assume the case. In addition, the transparent substrate 11, the first light reflection layer 12, the transparent resin layer 14, and the second light reflection layer 15 actually have different refractive indices, but here, for the sake of simple explanation, first, Assume that all have the same refractive index n.
[0018]
The reading light is focused on either the first or second information recording surface via the objective lens in order to read the recorded information. For example, a red semiconductor laser is used as a reading light source, and the laser beam is focused to a beam spot of about 1 μm via an objective lens. In order to focus on the second information recording surface, for example, an operation is performed in which the objective lens is once sufficiently moved away from the disk and then gradually moved closer to the disk. In such an operation, a focus error signal obtained based on the reflected light of the read light (the focus error detection system is a well-known one, which will not be described in detail here) is transmitted between the objective lens and the disk as shown in FIG. 5 shows an S-shaped curve corresponding to each of the surface 11, the first information recording surface, and the second information recording surface of the substrate in accordance with the amount of displacement of the distance. Therefore, by applying the in-focus (pull-in operation) of the focus servo at the third S-shaped curve from the start of the focusing operation, the read light can be focused on the second information recording surface.
[0019]
For reference, when the reading light is focused on the second information recording surface, the size a of the reading light (laser light beam) spot on the first information recording surface is a geometrical shape as shown in FIG. It can be determined using a schematic diagram. This is the same as the spot size of the light beam on the second information recording surface when the reading light is focused on the first information recording surface, as can be seen from FIG.
[0020]
The beam spot diameter a and the distance t between the first and second information recording surfaces are:
[0021]
(Equation 5)
2t · tan θ ′ = a (1.1)
There is a relationship.
On the other hand, according to Snell's law,
[0022]
(Equation 6)
sin θ / sin θ ′ = NA / sin θ ′ = 1 / n (1.2)
It becomes. Here, n is the material between the first and second information recording surfaces, that is, the equivalent refractive index of the transparent resin layer 14 (when a plurality of layers exist between the first and second information recording surfaces, t is the information recording time). It is the total thickness between the surfaces, and n is the equivalent refractive index when multiple layers are optically replaced by one.)
[0023]
In FIGS. 5 and 6, the refractive index of the material between the transparent substrate 11 and the first and second information recording surfaces is set to be the same.
[0024]
(Equation 7)
sin θ / sin θ ′ = 1 / n ₁ (1.3)
[0025]
(Equation 8)
sin θ ′ / sin θ ″ = n ₁ / n ₂ (1.4)
By
[0026]
(Equation 9)
sin θ / sin θ ″ = 1 / n ₂ (1.5)
It is not always necessary that the refractive index is the same as that of the transparent substrate 11.
From equation (1.2),
[0027]
(Equation 10)
sin θ ′ = NA × n (1.6)
Because
[0028]
(Equation 11)
θ ′ = sin ⁻¹ (NA × n) (1.7)
It is.
By substituting equation (1.7) into equation (1.1),
[0029]
(Equation 12)
a = 2t × tan {sin ⁻¹ (NA × n)} (1.8)
It becomes.
The main feature of the present invention is that when a signal is read by focusing on a certain information recording layer with reading light, the other information recording layer is in a defocused state. ) And the distance between the information recording layers, the information recording layer distance and the band of the signal to be recorded may be determined so that the read component of the recording signal of another information recording layer does not become noise (item 1). ). Further, on the premise of this item 1, in consideration of the fact that the reproduction level at the time of defocusing drops sharply in the high frequency range, a signal is recorded on each recording surface by a method in which low frequency components are suppressed (item 2), and each information In consideration of the degree of modulation of the read signal by the recording surface, the reflectance of the information recording surface closer to the incident side of the reading light is made smaller than that of the information recording surface farther (item 3), and the S / N is further improved. In order to achieve this, the reflected light band of each information recording surface should be different (item 4), and considering the ease of material selection, two types of wavelength light should be applied to the information recording surface closer to the incident side of the reading light. Use a material that shows a large reflectance only when irradiated simultaneously (item 5), and arrange a liquid crystal layer between the information recording surfaces in order to improve the S / N without depending on the material of the light reflection layer. When reading the information recording surface closer to the side where the reading light Blocking the incidence of the reading light on the information recording surface (item 6), and determining the spiral direction of the recording track on each recording surface in view of the smooth execution of the reading operation including focus control and tracking control in the reproduction system. (Item 7) Recording TOC information on all information recording surfaces in consideration of random access in the reproduction system (Item 8). Further, the present invention focuses on the fact that the energy density of the reading light differs depending on the information recording layer, and forms the information recording layer with a light reflecting material that generates return light at a light amount or wavelength according to the energy density of the reading light. (Item 9) is presented.
[0030]
Hereinafter, each of these items will be described in order.
First, items 1 and 2 will be described.
As shown in FIGS. 5 and 6, when one of the two stacked information recording surfaces is focused and irradiated with the reading light, the reading light is defocused and the other information recording surfaces are defocused. (That is, all reflected light by the read light has a reflection component at the time of defocusing according to the record information on the other information recording surface). FIG. 8 shows the MTF characteristics in the defocused state and the MTF characteristics in the on-focus state. The MTF characteristic is based on the OTF characteristic according to the imaging theory for a disk having an optical transmission characteristic in which the reading light is incident and reflects it.
[0031]
In FIG. 8, the read signal (reflected light) of the information recording surface on which the reading light is focused to read the recorded information follows the on-focus curve (thick line), and the read signal of the other information recording surface which is not to be read is obtained. (Reflected light) follows a curve (thin line) at the corresponding defocus.
According to the characteristic curve formed by such a thin line, even when defocusing is performed with defocus aberration W ₂₀ ^rms = 0.1λ, 0.2λ, 0.3λ,... And lifts a little at high frequencies. Since the reproduction level at the time of defocusing is for an information recording surface (information recording surface in a defocused state) other than the information recording surface to be read (information recording surface in an on-focus state), The smaller the playback level, the better.
[0032]
In order to make the reproduction level at the time of defocus less than, for example, 1/10 of the reproduction level at the time of on-focus, the normalized spatial frequency, that is, the frequency band of the recording signal is limited in accordance with the defocus aberration amount. It looks like a graph. That is, signals (pits) may be recorded on the information recording surface by narrowing down to a band (limited band) whose upper limit is the normalized spatial frequency indicated by the curve fmax in the graph and whose lower limit is the normalized spatial frequency indicated by the curve fmin. On the other hand, in order to make the reproduction level in defocus 1/10 or less of the reproduction level in on-focus over the entire range of 0.2 to 1.2 rmsλ of the defocus aberration, the band of the normalized spatial frequency must be 0.7-1. 4 is required.
[0033]
Such narrowing is performed by forming a recording signal using a run-length limited (RLL) code for suppressing a DC component or shaping the recording signal band using a partial response for limiting a transfer characteristic in order to obtain such a limited band. Or a method of forming a recording signal by carrier modulation such as FM, PM, QAM, QPSK, and VSB.
[0034]
Thus, it can be seen that the bandwidth of the usable recording signal can be set by setting the defocus aberration to 0.2 rmsλ or more.
Next, the upper and lower limits of the distance between the first information recording layer and the second information recording layer when the defocus aberration is set to 0.2 rmsλ or more are obtained.
The light component reflected by the defocus information recording surface in the lower limit read signal is defocused from the on-focus information recording surface by an amount corresponding to the first and second information recording interlayer distances t (see FIGS. 5 and 6). It almost follows the OTF at the time. The defocus aberration W ₂₀ ^{rms at} this time is
[0035]
(Equation 13)

[0036]
[Equation 14]

[0037]
It is. In order to sufficiently suppress the reflected light component of the defocused information recording surface with respect to the reflected light component of the on-focus information recording surface, at least W ₂₀ ^rms > 0.2λ is indispensable as described above.
[0038]
(Equation 15)

[0039]
(Equation 16)

[0040]
[Equation 17]
∴ t> 1.39 nλ / NA ² (1.13)
It becomes.
If n = 1.5,
[0041]
(Equation 18)
t> 2λ / NA ² (1.14)
It is.
For example, the recording signal is a (1,7) modulation code by 0,1 coding in which seven or more 0s are not consecutive, and a shortest inversion interval 2T (T is a sample period) is, for example, 1.2 in a standardized spatial frequency and a maximum inversion Assuming that the interval 8T is の of 0.3, which is １ ／ of that, from FIG. 9, if the defocus aberration amount is 0.6 rmsλ or more, the on-focus can be achieved in the normalized spatial frequency band of 1.2 to 0.3. It is satisfied that the reproduction level at the time of defocus becomes 1/10 or less of the reproduction level of. Assuming that W ₂₀ ^rms = 0.6λ, n = 1.5, λ = 680 nm, and NA = 0.55, the thickness t between the first and second information recording layers is 14 μm from the above equation (1.10). It turns out that it is good if it is above.
[0042]
When W ₂₀ ^rms = 0.2λ, and n = 1.5, λ = 680 nm, and NA = 0.55, the thickness t between the first and second information recording layers is obtained from the above equation (1.10.). , A lower limit of 4.67 μm is derived, but considering the in-focus capture range of the focus servo, it is desirable that the thickness t be larger than 10 μm.
[0043]
That is, an error signal is included in the focus error signal due to the reflected light from the defocus information recording surface. To sufficiently reduce the focus error signal, about three times the capture range CR as shown in FIG. 4 is required. . Normally, the capture range must be at least 3-4 μm or more,
[0044]
[Equation 19]
Ｔ t ≧ 10 μm (1.15)
It becomes.
It is preferable to adopt the larger one of the lower limit defined by the equation (1.15) and the lower limit defined based on the equation (1.10.).
[0045]
If the distance t between the information recording surfaces is too large, the spherical aberration increases and the recording signal cannot be read, so the upper limit of the thickness t is derived.
That is, in the disc having such i (i is an integer of 2 or more) pieces of information recording surface in FIG. 10, in the case to read and when read first information recording surface S ₁ of the i-th information recording surface S _i, to light The thickness differs by (i-1) t (t is the distance between recording surfaces, and the distance between each recording surface is equal). At this time, spherical aberration occurs.
[0046]
Now, the spherical aberration using the corrected lens, the first information recording surface S ₁ or the i information recording surface with respect to the midpoint between the first information recording surface S ₁ and the i-th information recording surface S _i i number of spherical aberration generated in the recording layer of each layer when reading the S _i is
[0047]
(Equation 20)

[0048]
It becomes. It is empirically known that when reproducing an ordinary optical disc, it is necessary to set the wavefront aberration value to be smaller than a martial crylarion (0.07 rmsλ). Since it is necessary to consider the aberration of the objective lens and the aberration due to the tilt of the disk in the wavefront aberration, the allowable amount of W ₄₀ ^rms needs to be less than １ ／ of about 0.07 ^rms λ.
[0049]
(Equation 21)

[0050]
(Equation 22)
^{∴ t ≦ 7.5n 3 λ / {} (n 2 -1) NA 4 (i-1)} ... (1.18)
It is.
[0051]
Now, if n ≒ 1.5,
[0052]
(Equation 23)
t ≦ (20 / NA ⁴ ) {λ / (i−1)} (1.19)
And if i = 2, then
[0053]
(Equation 24)
t ≦ 20λ / NA ⁴ (1.20)
It becomes.
Therefore, according to the equations (1.14) and (1.20), the distance t between the information recording surfaces when n = 1.5 and i = 2 is
[0054]
(Equation 25)
2λ / NA ² ≦ t ≦ 20λ / NA ⁴ (1.21)
It is determined. Based on this equation, the lower limit and the upper limit of t are obtained by changing NA and λ with practical values that are relatively easy to adopt, as shown in the table of FIG. Further, the equation (1.15) is added to this condition,
[0055]
(Equation 26)
10 μm ≦ t ≦ 20λ / NA ⁴ (1.22)
Is preferably satisfied.
Thus, when the item 1 is summarized, as described in the equations (1.13) and (1.18),
[0056]
[Equation 27]
1.39 nλ / (NA) ² ≦ t ≦ 7.5 n ³ λ / {(n ² −1) (NA) ⁴ (i−1)} (I)
By manufacturing a multi-layered disc satisfying the above condition, the recorded information on the focus information recording surface can be read without being affected by the defocus information recording surface. And more preferably, in addition to the above formula,
[0057]
[Equation 28]
10 μm ≦ t ≦ 7.5 n ³ λ / {(n ² −1) (NA) ⁴ (i−1)} (II)
By satisfying the above condition, it is possible to maintain the focus error signal based on the capture range that is the width of the pull-in operation in the focus servo.
[0058]
Items 1 and 2 do not require that each of the information recording surfaces has a different reflected light band.
Next, item 3 will be described.
This is applied to a case where a film whose reflectance and transmittance do not change depending on the wavelength of the incident reading light is used as a reflection film of each information recording surface.
[0059]
For example, in FIG. 1, when reading the second information recording surface (reflective layer 15), light transmitted through the first information recording surface (reflective layer 12) is reflected by the second information recording surface, and the reflected light Further, light that has passed through the first information recording surface and returned to the light source side as reflected light of the disk is received, and a read signal corresponding to the amount of received light is obtained. Assuming that each reflective layer does not absorb light, the reflectance of the first information recording surface is α, the transmittance is (1-α), the reflectance of the second information recording surface is β, and the transmittance is (1-β). ), The reading light amount is P, and the light transmitted through the second information recording surface is assumed not to return almost as reflected light of the disk. If the modulation degree of each information recording surface due to pits is k, the first information recording surface is assumed to be k. The modulation degree M1 of the read signal obtained when reading the surface is
[0060]
(Equation 29)
M1 = Pαk / {α + (1−α) ² β}
= Pαk / (α + β- 2αβ + α 2 β)
≒ Pαk / (α + β) (3.1)
The modulation degree M2 of the read signal obtained when reading the second information recording surface is:
[0061]
[Equation 30]
^{M2 = P (1-α)} 2 βk / {α + (1-α) 2 β}
≒ P (1−α) ² βk / (α + β) (3.2) Both α and β are numbers smaller than 1, and (1−α) ² is a square value of a number smaller than 1. Therefore, in order for Expressions (3.1) and (3.2) to have similar values, β needs to be sufficiently larger than α. That is, when viewed from the side where the reading light is incident, it is necessary to set the reflectance of the far information recording surface to be larger than that of the near information recording surface.
[0062]
For example, if α = 0.36 and β = 0.9,
[0063]
[Equation 31]
M1 ≒ 0.36Pk / 1.26 (3.3)
[0064]
(Equation 32)
M2 ≒ 0.37Pk / 1.26 (3.4)
Thus, M1 and M2 are substantially equal. FIG. 12 shows the reflectance characteristics of each information recording surface in this case. The horizontal axis λ represents the wavelength of the reading light, and the vertical axis R represents the reflectance by the reading light.
[0065]
Note that also in item 3, it is not required that each of the information recording surfaces has a different reflected light band.
Next, item 4 will be described. Here, it is required that each of the information recording surfaces has a different reflected light band.
In the embodiment described with reference to FIG. 12, the conditions of the reflective film when reading a plurality of information recording surfaces using a single-wavelength reading light source are shown. However, ideally, when reading one information recording surface, The other information recording surface is desirably colorless and transparent. If a plurality of wavelengths of the reading light source are used, and each reflecting film reflects light corresponding to these wavelengths, a state close to this can be achieved. However, since it cannot be completely colorless and transparent, it is desirable to satisfy the above formula (I), preferably (II), even in this case.
[0066]
Therefore, after satisfying the formula (I) or (II), as shown in FIG. 13, the first information recording surface S ₁ is read light of wavelength λ ₁ and the second information recording surface S ₂ is in the wavelength lambda ₂ of the read light, third information surface S ₃ shall be read by the reading light of wavelength lambda _3. That is, as shown in FIG. 14, the reflective film of the first information recording surface shows a large reflectance at a wavelength lambda _1, the lambda _2, lambda _3, down the reflectance, increases the transmittance, the second The reflective film on the information recording surface shows a large reflectance at the wavelength λ ₂ , the reflectance decreases at λ ₁ and λ ₃ , the transmittance increases, and the reflective film on the third information recording surface at the wavelength λ ₃ The characteristics are set so that the reflectivity is large, and the reflectivity decreases and the transmittance increases at λ ₁ and λ ₂ . As a result, a read signal with a good S / N can be obtained. It should be noted that a light reflective material having wavelength selectivity is employed for such a reflective film, for example, using a dye material or the like.
[0067]
Next, item 5 will be described. Here, it is required that the information recording surface closer to the read light incidence side among the plurality of information recording surfaces has a predetermined reflectance characteristic.
In the embodiment described with reference to FIGS. 13 and 14, the case where the reading light has a plurality of wavelengths is shown. However, the wavelength of the reading light is set to one type, and the light source other than the reading light uses a light source for the information recording surface. The recorded information may be read while changing the reflectance characteristics of the reflection film.
[0068]
For example, in the photochromic material, as shown in FIG. 15, the single incident light having a wavelength lambda _2, there is no change in reflectivity, when irradiated with light of wavelength lambda ₁ in addition to the wavelength lambda ₂ incident light , there is the reflectance increases at the wavelength lambda ₂ and the front and rear of a wavelength. Therefore, as shown in FIG. 16, the reflective film 12B on the information recording surface closer to the transparent substrate 11 is made of a photochromic material. For example, when a red laser diode is used as a reading light source and a green light source is irradiated as additional light, a photochromic layer serving as an information recording surface reacts to produce a disk whose reflectance in red increases.
[0069]
As shown in FIG. 17, such a recorded information reproducing system for a disk is provided with only one pickup 31 for irradiating the disk with a red laser beam reading light to detect information, and emits green light separately from this pickup. Lamp 32 is provided. When reading the first information recording surface (12B), the lamp 32 is turned on to irradiate the disk 33 (at least the information detection point of the pickup 31 and its vicinity) manufactured as described above, and the second information recording surface (12B). When reading (15), the green lamp 32 is turned off.
[0070]
Also, as shown in FIG. 18, a green light emitting diode 310 is provided inside the pickup 31A, and the diode output light is combined with the red reading light emitted by the red laser diode 311 for obtaining an information detection output. Alternatively, the disk 33 may be irradiated.
In FIG. 18, the emission light of the information-detecting red laser 311 passes through the green reflector 312 and enters the disk 33 via the half mirror 313 and the objective lens 314. The red light reflected from the disc 33 is reflected by the half mirror 313 and guided to the photo detector 317 via the red filter 315 and the optical system 316. The optical system 316 includes, for example, a parallel plate and a lens, and guides light transmitted through the filter 315 to the photodetector 317 to collect the light. On the other hand, the emission light of the green light emitting diode 310, which is the additional light source, enters the green reflector 312 via the lens 318 and is reflected there, and enters the disk 33 together with the red light from the laser 311 transmitted through the reflector 312. .
[0071]
When reading the first information recording surface, the driving circuit causes the green diode 310 to emit light, and the disk 33 is irradiated with both red and green wavelength light, and the reading operation is performed in the state indicated by the dotted line in FIG. When reading the second information recording surface, the green diode 310 is turned off, the disk 33 is irradiated with only red wavelength light, and the reading operation is performed in the state indicated by the solid line in FIG. Therefore, the green diode 310 is driven on and off corresponding to the information recording surface to be read.
[0072]
Further, a reproducing system having a high read transfer rate may be provided by using a plurality of pickups having a configuration such as the pickup 31A. For example, two such pickups are prepared, and one of the pickups is operated for reading the first information recording surface, and the other is operated for reading the second information recording surface, and the respective red laser and green diode are operated. As a result, both information recording surfaces can be read at the same time, and a lot of information can be read per unit time, which is suitable for a system handling a large amount of recorded information. In such a configuration, the information recording surfaces to be read by one and the other pickups can be reversed if necessary.
[0073]
Although the reading light and the additional light are described as being red light and green light, they may not be visible light, but may be infrared light or ultraviolet light. May be changed. The additional light source is not limited to a photodiode or a lamp, and may be a laser diode. The emission light of the diode 310 in FIG. 18 does not need to be focused on the information recording surface of the disk 33 as much as the laser light, and therefore does not impose a load on an optical component such as a lens. Further, since it is desirable that only the read light component of the recorded information is incident on the photodetector 317, a filter 315 that transmits only red light is interposed in the front stage, thereby improving the read efficiency.
[0074]
Next, item 6 will be described. Here, it is not required that each of the information recording surfaces has a different reflected light band.
In the embodiment described with reference to FIGS. 15 to 18, the reflection layer whose reflectance characteristic changes by irradiating the reading light with the additional light is used. However, as shown in FIG. 19, the liquid crystal layer sandwiched between the transparent electrodes is used. Thus, the reflectivity of each information recording layer may be changed.
[0075]
In FIG. 19, a transparent and conductive reflective layer 51 is laminated on a transparent substrate 11 as a first information recording layer, and this reflective layer 51 is disposed on the outer surface of the disk 50, for example, near the outermost periphery. It is connected so as to be electrically conductive with the conductor ring 51r. A transparent electrode layer 52 is formed on the reflective layer 15 as the second information recording layer and the transparent resin layer 14 laminated on the protective layer 16. A liquid crystal layer 54 surrounded by a space 53 is formed between the pair of transparent electrode layers 51 and 52. The conductor ring 52r is spaced apart from the conductor ring 51r on the inner peripheral side, and each ring is exposed.
[0076]
When reading and reproducing the disk 50, the disk is rotated while the brush is in contact with the exposed surface of each ring. Thus, when reading the first information recording surface, the drive circuit 57 darkens the liquid crystal layer 54 between the brush 55 contacting one conductor ring 51r and the brush 56 contacting the other ring 52r. The reading light is made incident while applying the appropriate driving voltage. Accordingly, the dark liquid crystal layer 54 blocks the input of the reading light to the second information recording layer 15, so that only the first information recording surface is read. When reading the second information recording surface, the drive circuit 57 is turned off and the liquid crystal layer 54 becomes bright, so that the reading light reaches the second information recording layer and the second information recording surface can be read.
[0077]
With this configuration, when reading the first information recording surface, there is no reading light incident on the second information recording surface, so that unnecessary reflected light components due to the recording signal of the second information recording surface are not generated. Can be read, and a further improvement in S / N is achieved.
Next, item 7 will be described.
[0078]
In particular, when reading a plurality of information recording surfaces with a single pickup, the following points should be considered.
On one information recording surface of the disc, signals are recorded from the inner periphery to the outer periphery, for example, in a spiral track shape, and the last signal recorded on the information recording surface is assumed to continue to another recording information recording surface. If the direction of the spiral on the information recording surface is the same and the rotation direction of the disc is not changed, as shown in FIG. 20, when the operation shifts to the reading operation of another information recording surface to follow the last signal. It is necessary to return to the inner circumference and refocus. Also, if it is CLV, the number of revolutions of the disk must be changed extremely. On the other hand, after reading the last signal, the disk must be rotated in the reverse direction in order to focus again on the outer periphery and to perform another operation for reading the information recording surface from the outer periphery.
[0079]
In view of such a defect, as shown in FIG. 21, for example, the spiral direction of the recording track is reversed between the first information recording surface and the second information recording surface, and from the inner periphery to the outer periphery on the first information recording surface. After the signal is read and the last signal is read, refocusing is performed without changing the rotation direction and the number of rotations of the disk, and the operation proceeds to the reading operation of the second information recording surface and the reading is performed from the outer periphery to the inner periphery. preferable. By doing so, it is possible to maintain smooth continuity of the reading operation of the multilayer structure disk. In the case of a disc having three information recording layers, such continuity can be maintained if at least one layer has an inverse spiral.
[0080]
When there are two information recording layers, taking into account ease of focus, reading on the first information recording surface starts at the outer periphery, proceeds to the inner periphery, and reads the last signal on the information recording surface. After that, it is preferable to re-focus and read the second information recording surface from the inner periphery to the outer periphery. In addition, the present invention is applicable not only to spiral tracks but also to concentric tracks.
[0081]
Next, item 8 will be described. Here, the ease of random access of the reproduction system is taken into consideration.
[0082]
One of the plurality of information recording surfaces is defined as an initial surface, and allocation information (information equivalent to TOC information used in a normal CD) on all recording information of the disc is provided in a predetermined recording area of the initial surface. , Hereinafter referred to as TOC information). FIG. 22 shows this, for example, the first information recording layer is set on the initial surface, and TOC information is recorded at the head of the recording area. In the TOC area, the TOC information on all the information recording layers of the first to m-th discs is individually recorded.
[0083]
By pre-reading the TOC information recorded in this way at the time of starting the reading operation, it is possible to respond quickly to random access specified later.
As another method, as shown in FIG. 23, a TOC area is provided in each of the information recording layers, and TOC information on all the information recording layers of the disc from the first to the m-th is recorded in this area. You may do it. According to this, the TOC information of the entire disc can be read from any information recording surface without particularly setting the initial surface.
[0084]
Before explaining matters 9, with reference to FIG. 10 before, the focus on the information recording surface S _i of the reading light to the substrate 11 and the information recording surface S ₁ ~S _i-1 to read is transmitted through the information recording surface S _i Let us consider the case (A) where control is performed. In this case, the return light reflected by the information recording surface S _i is guided to a light receiving system (not shown) via the substrate 11 through the information recording surface S ₁ ~S _i-1 again. Here, 100% reflectance of the bottom layer of the information recording surface S _i, when the transmittance and the transmittance between the information recording surface between the substrate 11 and the information recording surface S ₁ and α, respectively, from the substrate 11 The amount of the returning light emitted is represented by ^α2i . For example, when the transmittance is 90% and the number of stacked information recording surfaces is 10, α = 0.9 and i = 10, so that only 12% of the return light amount is 100% of the read light amount incident on the disk. I can't get it. While the reflectance of the information recording surface S _i in this trial was 100%, if the information recording surface S _i has a similar light reflection and transmission characteristics and other data recording surface, the light quantity of return light emanating from the substrate 11 Is represented by α ^i-1 (α-1) α ^i-1 . In this case, if α = 0.9 and i = 10, only 1.5% of the return light amount can be obtained with respect to 100% of the read light amount incident on the disk. As described above, in the multi-layer structure of the recording surface, it is necessary to increase the transmittance of each intermediate information recording layer in order to transmit the reading light to the lowermost information recording surface, while increasing the transmittance of the information recording layer. If the height is increased, there is a contradiction that the reading light cannot be sufficiently reflected on the information recording surface to be read, and a sufficient amount of return light cannot be obtained.
[0085]
Therefore, as a result of searching for a method of obtaining a sufficient return light amount only on the information recording surface to be read, that is, the information recording surface on which the focus control of the reading light is performed, the energy densities of the reading light are different on the information recording surface, and the reading light Attention was paid to the fact that the energy density of the reading light was the maximum on the information recording surface on which the focus control was performed. For example, the information recording surface distance t = 200 [mu] m, numerical aperture NA = 0.5 objective lens, and the refractive index n = 1.5 between the information recording surface, aligned the focal point of the read light on the information recording surface _{S 3} If (B) in which the diameter of the reading light spot on the information recording surface _{S 2} becomes approximate calculation by 200 ÷ 1.5 × 0.5 × 2 = 132 [μm]. Since the reading light spot on the focal point is about 1 [mu] m, the energy density of the read light on the information recording surface S ₂ becomes 1/100 or less of the information recording surface S _3, after all, the energy of the light reading in the adjacent information recording surface It can be seen that there is a considerable difference in density.
[0086]
In this matter, using a difference in energy density of the read light between the information recording surface, the reflective layer forming the information recording surface S ₁ to S _i, employing a material showing a reflection factor corresponding to the light energy density I do. In other words, when the reading light is focused and the light energy density is maximized, the reflectance increases significantly, and in other cases, the information recording film is formed of a light reflecting material having a property of having a relatively high transmittance. You do it. Examples of such an information recording film include a photochromic film, a thermochromic film, and a phase change film. A desired information recording surface can be read with a single reading light for a multi-layer recording disk in which each information recording surface is formed by such an information recording film. The information recording surface at the focus position of the read light reflects the incident read light with a high reflectance due to the above-mentioned properties, while the information record surface not at the focus position of the read light reflects the incident read light with a high transmittance. , The return light of the disc probably contains a component modulated by the information recording surface at the in-focus position, and the component modulated by the information recording surface not at the in-focus position is extremely small. Therefore, it is possible to satisfactorily obtain a read signal of the information recording surface to be read from such return light.
[0087]
Materials In the present matter, indicating using a difference in energy density of the light reading between the information recording surface, the reflective layer forming the information recording surface S ₁ to S _i, the reflection band corresponding to the light energy density Is adopted. In other words, the wavelength conversion property reflects the wavelength component different from the reading light when the reading light is focused and the light energy density is maximized, and otherwise reflects the wavelength component equivalent to the reading light. The information recording film is formed by the light reflecting material. Examples of such an information recording film include a photoluminescence film, a fluorescent dye film, and an optical semiconductor film. A desired information recording surface can be read with a single reading light even for a multi-layer recording disk in which each information recording surface is formed by such an information recording film. The information recording surface at the focus position of the reading light reflects light of the second wavelength different from the incident reading light due to the above-mentioned properties, while the reading light incident at the information recording surface not at the focusing position of the reading light. Since the light of the same first wavelength is reflected, the return light of the disc has the second wavelength light modulated by the information recording surface at the in-focus position and the first light modulated by the information recording surface not at the in-focus position. Wavelength light. Therefore, the second wavelength light can be extracted from such return light and obtained as a read signal of the information recording surface to be read.
[0088]
FIG. 24 shows an example of a reproduction system including a means for extracting the second wavelength light, and portions equivalent to those in FIG. 18 are denoted by the same reference numerals.
In Figure 24, the optical disk 33 'the material is adopted showing the reflection band corresponding to the light energy density on the reflective layer to form a such as mentioned above the information recording surface _{S 1, S 2, S 3} , from the light source 311' A laser beam having a wavelength λ1 is irradiated as reading light via a half mirror 313 and an objective lens 314. The reading light to the optical disk 33 'is focus control to focus on the information recording surface S ₂ in the middle, the light energy density is increased in the information recording surface S _2, the wavelength λ1 incident on the information recording surface S ₂ some components of the reading light where it is converted into the reflected light of the wavelength lambda _2. At the same time, the other components of the reading light having the wavelength λ1 reach the information recording surfaces S ₁ and S _3. However, since the light energy density is extremely low, the reading light is reflected at the wavelength λ1. Thus from the disc 33 'of the return light is obtained returning light and the wavelength lambda ₂ wavelength .lambda.1, it is guided to the optical filter 320 through the objective lens 314 and the half mirror 313. Filter 320 is responsible for extracting means for transmitting only light components having a wavelength lambda _2, blocking the return light having a wavelength λ1 from the half mirror 313, and supplies the return light only the wavelength lambda ₂ to the photodetector 317. Thus, detector 317 can receive only the return light modulated by the recording signal of the information recording surface S ₂ to be read. The detector 317 supplies an electric signal corresponding to the amount of received light to a signal processing system (not shown) as a read signal.
[0089]
In FIG. 24, the optical path is schematically depicted in a form different from the actual one in order to clearly show the state of the change in the wavelength. Further, the objective lens 314 and half mirror 313, it is preferable to adopt those having equivalent optical characteristics for both wavelengths λ1 and the wavelength lambda _2. Further, the disc having an information recording layer that performs wavelength conversion according to the light energy density is not limited to the reflection type as described above, and can be applied to a transmission type.
[0090]
【The invention's effect】
As described in detail above, according to the multi-layer recording disk of the present invention, the read light is not reflected on the information recording surface with respect to the reproduction level obtained when the read light is on-focused on one of the information recording surfaces. The information signal is recorded on each of the information recording surfaces in a frequency band in which the reproduction level obtained when the other information recording surface is defocused is 1/10 or less, and the distance between the information recording surfaces is t, the numerical aperture NA of the objective lens that guides the reading light to any of the information recording surfaces, the wavelength λ of the reading light, the equivalent refractive index n between the information recording surfaces, and the number i of stacked information recording surfaces,
[0091]
[Equation 33]
^{1.4nλ / (NA) 2 ≦ t} ≦ 7.5n 3 λ / {(n 2 -1) (NA) 4 (i-1)}
And at least one information recording surface of each of the information recording surfaces has information recorded on the spirally formed recording track from the disk outer periphery to the inner periphery, and the other information recording surface The information is recorded from the inner circumference to the outer circumference of the disk on a spirally formed recording track in a different spiral direction from the recording track formed on the first information recording surface . As a result, smooth continuous reading from the multilayer recording disk becomes possible, and a defocus aberration of not less than a predetermined value and a spherical aberration of not more than a predetermined value are maintained. And a multi-layer disc capable of recording and reading the recorded signal and reproducing the same, and a recording system and a reproduction system using the same.
[Brief description of the drawings]
FIG. 1 is a sectional view showing the structure of a ROM-type optical disc having two light reflection layers as information recording layers in an embodiment according to the present invention.
FIG. 2 is a cross-sectional view of the disk in each step for explaining the method of manufacturing the optical disk in FIG.
FIG. 3 is a sectional view showing the structure of a ROM type optical disk having a structure equivalent to the optical disk in FIG. 1;
FIG. 4 is a graph showing a focus error level with respect to a displacement amount of a distance between an objective lens and a disk in a focusing operation on the optical disks of FIGS. 1 and 3;
FIG. 5 is a geometric diagram showing the size of the light beam on the first information recording surface when the light beam is focused on the second information recording surface in the optical discs of FIGS. 1 and 3;
FIG. 6 is a geometric diagram showing the size of the light beam on the second information recording surface when the light beam is focused on the first information recording surface in the optical discs of FIGS. 1 and 3;
FIG. 7 is a diagram showing an optical disk shown in FIGS. 1 and 3 in which the refractive index of the transparent substrate and the layer between the first and second information recording surfaces are not the same, 1 is a geometric diagram showing the size of a light beam on an information recording surface.
FIG. 8 is a graph showing MTF characteristics of the optical disks of FIGS. 1 and 3 in a defocused and on-focus state of a light beam;
FIG. 9 is a graph showing the maximum value and the minimum value of the normalized spatial frequency with respect to the amount of defocus aberration when the reproduction level at the time of defocus is set to 1/10 or less of the reproduction level at the time of on-focus.
FIG. 10 shows an optical disc according to the present invention having i information recording surfaces for explaining an upper limit value of a distance between information recording surfaces and explaining a difference in light energy density of each information recording surface of read light. FIG.
FIG. 11 is a table showing a calculation example of upper and lower limits of a distance between information recording surfaces in an optical disc according to the present invention.
FIG. 12 is a graph showing the reflectance characteristics of one information recording surface and the other information recording surface of the optical disk according to the present invention which are closer to the side where the reading light is incident.
FIG. 13 is a simplified cross-sectional view of a disc showing a manner in which first to third information recording surfaces are read by reading light beams having different wavelengths.
14 is a graph showing the reflectance characteristics of each information recording surface of the optical disc in FIG.
FIG. 15 is a graph showing reflectance characteristics of a photochromic material.
FIG. 16 is a sectional view showing a multilayer recording disk according to an embodiment of the present invention.
FIG. 17 is a block diagram showing a configuration of a reproducing system according to an embodiment of the present invention for reproducing the information recorded on the disc in FIG. 16;
FIG. 18 is a block diagram showing the configuration of a reproduction system according to another embodiment of the present invention for reproducing the information recorded on the disc in FIG.
FIG. 19 is a sectional view and an external view of a multilayer recording disk having a liquid crystal layer according to an embodiment of the present invention.
FIG. 20 is a schematic diagram for explaining a defect in a spiral direction of a recording track on each information recording surface in a multilayer recording disk.
FIG. 21 is a schematic diagram for explaining a spiral direction of a recording track on each information recording surface in the multilayer recording disk according to the present invention.
FIG. 22 is a schematic diagram showing an example of a recording form of TOC information on a multilayer recording disk according to the present invention.
FIG. 23 is a schematic view showing another example of the recording form of the TOC information on the multilayer recording disk according to the present invention.
FIG. 24 is a block diagram showing a configuration of a reproducing system for reading a recording signal of a multilayer disc according to the present invention.
[Description of Signs of Main Parts]
11,11A transparent substrate 12,12A first reflective layer 13 radiolucent transparent stamper 14 ultraviolet curing resin layer 15 and the second reflective layer 16 protective layer _S 1 to S _i information recording surface 12B photochromic reflective film 31 and 31A, 31A ' Pickup 310 Green LED
311 Red laser 312 Green reflector 313 Half mirror 314 Objective lens 315 Red filter 316 Optical system 317 Photodetector 318 Lens 50 Multilayer recording disk 51, 52 Transparent electrode layer 51r, 52r Conductor ring 53 Space 55, 56 Brush 57 Drive circuit 311 ' Reading light source 320 Optical filter

Claims (22)

A multilayer recording disc in which a plurality of information recording surfaces on which information is read only from one surface side of the disc surface are stacked,
A distance t between each of the information recording surfaces, a numerical aperture NA of an objective lens for guiding read light for reading recorded information from the multilayer recording disk to any of the information recording surfaces, a wavelength λ of the read light, the information recording surface The equivalent refractive index n between each of them and the number i of stacked information recording surfaces are
## EQU1 ## 1.4nλ / (NA) ² ≦ t ≦ 7.5n ³ λ / {(n ² −1) (NA) ⁴ (i−1)}
Satisfies
The read light is defocused on another information recording surface of the information recording surfaces with respect to a reproduction level obtained when the read light is on-focused on one of the information recording surfaces. An information signal is recorded on each of the information recording surfaces in a frequency band in which the obtained reproduction level is 1/10 or less,
On at least one of the information recording surfaces, information is recorded from a disk outer periphery toward an inner periphery on a spirally formed recording track, and the other information recording surface has the above-mentioned information recording surface. The information recording is performed from the inner periphery to the outer periphery of the disk on a spiral recording track having a different spiral direction as viewed from the one surface side with respect to the recording track formed on the information recording surface of the disc. Features a multilayer recording disk. The distance t between each of the information recording surfaces is further:
[Number 2] ^{10μm ≦ t ≦ 7.5n 3 λ /} {(n 2 -1) (NA) 4 (i-1)}
2. The multi-layer recording disk according to claim 1, wherein: A recording system for recording an information signal on the multilayer recording disc according to claim 1,
A recording system for performing recording on the information recording surface within the band using an information signal formed by a run-length limited code. A recording system for recording an information signal on a multilayer recording disk according to claim 1,
A recording system for performing recording on the information recording surface within the band using an information signal formed by a partial response system. A recording system for recording an information signal on a multilayer recording disk according to claim 1,
A recording system for performing recording on the information recording surface within the band using an information signal formed by a carrier modulation method. The recording system according to claim 5, wherein the carrier modulation method is an FM modulation method. 2. The multi-layer recording disk according to claim 1, wherein the information recording surface is formed of a light reflecting layer, and has a lower reflectance as it is closer to a side where the reading light is incident. 2. The multi-layer recording disk according to claim 1, wherein the information recording surface is formed by a light reflection layer, and has different reflectances for a predetermined wavelength. A reproduction system for reproducing a recording signal of a multilayer recording disk according to claim 8, wherein:
A reproducing system, which irradiates read light having a wavelength corresponding to an information recording surface to be reproduced. The information recording surface includes at least two information recording surfaces, and a reflectance of the information recording surface of the information recording surface closer to a side where the reading light is incident with respect to the reading light has a first wavelength. 2. The case where the reading light and the additional light having the second wavelength different from the first wavelength are simultaneously irradiated, the irradiation light is larger than the case where only the light is irradiated. 3. Multi-layer recording disc. A reproduction system for reading the multilayer recording disk according to claim 10 with a single-wavelength reading light,
A reproducing system, comprising: an additional light irradiating unit that irradiates the additional light at least at a reading point when reading an information recording surface closer to a side where the reading light is incident among the information recording surfaces. 12. The reproduction system according to claim 11, further comprising a pickup for irradiating the reading light, wherein the pickup includes the additional light irradiation unit. 13. The reproducing system according to claim 11, wherein the additional light irradiating unit is controlled to turn on and off irradiation of the reading point according to an information recording surface to be read. The information recording surface includes at least two information recording surfaces,
A liquid crystal layer sandwiched between a pair of transparent electrode layers between the information recording surface closer to the side where the read light is incident and the other information recording surface of the information recording surface. 2. The multilayer recording disc according to 1 above. 2. The device according to claim 1, further comprising two ring-shaped conductors exposed on an outer surface of the disk on which the reading light is incident, wherein the pair of transparent electrode layers are individually connected to the ring-shaped conductors. 15. The multilayer recording disc according to 14. A reproduction system for reading a multilayer recording disc according to claim 15,
Two brushes that are in contact with each of the ring-shaped conductors, and via the brushes to darken the liquid crystal layer when reading the information recording surface of the information recording surface that is closer to the side where the reading light is incident. And a voltage drive circuit for applying a voltage to the ring-shaped conductor. A reproducing system for reading a multilayer recording disc according to claim 1, wherein
In response to the start command, reading starts from one of the inner and outer circumferential positions of one information recording surface, and after reading the last signal of the one information recording surface, the other signal is read at the other position. A focus operation of reading light on the information recording surface, and restarting reading from the other position on the other information recording surface. 2. The multi-layer recording disk according to claim 1, wherein management information on another information recording surface among the information recording surfaces is recorded on at least one of the information recording surfaces. 2. The multi-layer recording disk according to claim 1, wherein management information on all recording information recorded on the disk is recorded on one of the information recording surfaces. 2. The multilayer according to claim 1, wherein each of the information recording surfaces is formed by a light reflection layer, and the light reflection layer is made of a light reflection material having a reflectance according to an energy density of the reading light. Recording disc. 2. The multi-layer recording disk according to claim 1, wherein each of the information recording surfaces is formed of a material that converts the wavelength of the reading light according to the energy density of the reading light. 22. A reproducing system for reproducing a recording signal of a multilayer recording disk according to claim 21,
Reading light generating means for emitting reading light having a predetermined wavelength, and a light component having been subjected to wavelength conversion by the information recording surface among reflected or transmitted light obtained by making the reading light incident on the multilayer recording disk. A reproduction system comprising: an extraction unit for extracting; and a light receiving unit for receiving a light component extracted by the extraction unit and generating a read signal corresponding to the light component.

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